In marine seismic exploration, one or more seismic sources are deployed in a body of water and fired to produce sonic pulses or shock waves which propagate through the body of water and into the subterranean geologic formations beneath the floor of the body of water. The pulses are reflected back from the subfloor geologic formations as acoustic waves. An array of geophones, hydrophones, or like equipment detects the reflected acoustic waves and converts such waves to electronic signals. These electronic signals are recorded for subsequent analysis and interpretation. Analysis of the recorded signals can provide an indication of the structure of the subfloor geological formations and attendant petroleum accumulations in those formations.
Marine seismic exploration is most commonly conducted in bodies of saltwater, however, the term "water", as used in this description and in the appended claims, is meant to include sea, lake, swamp, and marsh water, mud, and any other liquid containing sufficient water to enable operation of the marine seismic sources employed in connection with the invention.
There are a number of conventional marine seismic sources available for generating a sonic pulse in a body of water. For example, explosives such as dynamite may be used to introduce strong pulses into subfloor formations. The use of such explosive charges has declined, however, due to safety and ecological concerns. Another conventional marine seismic source utilizes the discharge of a bank of capacitors through a subsurface electrode to produce a quickly collapsing implosive gaseous bubble. Yet another conventional marine seismic source uses explosive gases (such as mixtures of propane and air or propane and oxygen) to produce a sonic pulse on ignition. Implosive sources, such as water guns, are also conventional in marine seismic exploration.
Other, more common conventional acoustic energy sources use high pressure compressed gases instead of explosive mixtures. Typical designs for open ported compressed gas guns are found in U.S. Pat. No. 3,653,460 issued Apr. 4, 1972 to Chelminski and U.S. Pat. No. 4,141,431 issued Feb. 27, 1979 to Baird. A typical compressed gas gun for marine seismic exploration includes a cylindrical housing which contains a chamber adapted to confine a charge of compressed gas at high pressure. The chamber is fitted with a valve. The valve is closed while the pressure is increased in the chamber. When the gun is "fired", the valve is rapidly opened. This allows the compressed gas to expand out of the chamber and, through exhaust ports in the housing, into the surrounding medium to create an acoustic pulse.
A particular type of compressed gas gun, the air gun, has become widely used as a marine seismic energy source. The typical air gun has the compressed gas gun configuration described above wherein the compressed high pressure gas is air. Typically, the compressed air in such guns is maintained at pressures between 2,000 and 6,000 psi prior to release into the water to create the desired acoustic pulse.
Conventional air guns typically include a cylindrical housing containing exhaust ports through which the compressed air is released when a valve is opened in the gun. The exhaust port configuration of these underwater compressed air guns may vary. In a common configuration, a plurality of exhaust ports are distributed around the periphery of the cylindrical housing of the compressed gas gun. PAR.RTM. Air Guns available from Bolt Technology Inc., Norwalk, Conn. are examples of air guns with four symmetrically distributed exhaust ports. In another configuration, compressed air is released through a single exhaust port which extends 360.degree. about the periphery of the gun. Sleeve Guns.RTM. available from Geophysical Service, Inc., Dallas, Tex. are examples of air guns with single 360.degree. exhaust ports. In such external sleeve air guns, a shuttle valve concentric with the gun housing slides along the outer surface of the housing to open and close the exhaust port.
Although such air guns are widely used in industry and possess significant advantages over previously employed devices, their effectiveness in seismic exploration is frequently hampered by difficulties with secondary oscillations which are associated with the acoustic impulses they generate. It is well known to those skilled in the art that the preferred form of sonic energy for use in seismic exploration has the form of a single acoustic impulse, rather than a train or series of impulses as may be produced as a result of secondary oscillations. When explosive charges were more widely used in seismic exploration, such single impulses were achieved by firing the explosive charges near the surface of the body of water so as to vent the explosion to the atmosphere and thus preclude the generation of secondary oscillations. The strength of the pulses generated by air guns, however, is not great enough to permit their firing so near the water surface. Instead, it is necessary to fire the air gun at a reasonable depth, where there is much less loss of signal strength. When an air gun is fired at such depths, however, the discharged air forms a bubble, the elasticity of which couples with the inertial mass of the surrounding water to produce an oscillating system. The air bubble will grow and shrink at its natural period until the energy is dissipated to the water and the bubble comes to equilibrium volume. The oscillations so produced are undesirable because they produce a train of secondary pressure pulses which reduce the spectral quality obtainable from within the initial primary signal component. The amplitude ratio of the primary signal component of the generated signal to the strongest of the accompanying successive oscillation components has commonly been termed the "primary to bubble ratio". As is known to those skilled in the art, the primary to bubble ratio is maximized wherever possible, in order to optimize the spectral frequency content of the air gun.
It has been learned that the primary to bubble ratio may be improved by a particular positioning of the air guns. By increasing the primary to bubble ratio, the technique produces an acoustic signal much more suitable for seismic exploration. The technique for improving the primary to bubble ratio involves an array of at least three air guns. The air guns are adapted to produce discharged air bubbles each having a substantially equal maximum radius, R. Each air gun is separated from each of the other air guns nearest thereto by a certain critical distance, D, which is selected so as to maximize the primary to bubble ratio. D should not be less than 1.2 R and should not be greater than the quantity 2R. The sources in the array, the air guns, are characterized as "interdependent" when they are separated by the critical spacing. In one embodiment, the array includes a set of four air guns positioned at the corners of a square having sides of approximate length /2R. In another embodiment, the array includes a set of three air guns positioned at the vertices of a triangle. The guns in each array are fired simultaneously as a set. Two or more such arrays may be deployed simultaneously as a multiple array.
Practice of this technique requires that the desired critical distance between the air guns in the array be maintained while the array is deployed and fired. One approach which has been used is to arrange the air guns within a rigid boxlike positioning structure by means of chains attached unyielding to the gun bodies and structure. This arrangement, however, is difficult to assemble and, once it is assembled, it is difficult to remove and install individual guns should their repair or replacement be necessary. This approach also results in a bulky, unwieldly assembly which is difficult to handle on deck and deploy from a marine seismic vessel.
It is also central to the practice of the abovedescribed technique that the air guns in the array be fired together simultaneously. When fired, each of the air guns generates severe explosive shock energy which is potentially very destructive to the positioning structure to the gun itself and its attachment points, and, if transmitted through the positioning structure, to the other guns in the array. For example, when the boxlike positioning structure described above has been used, the unyielding mounted chains have suffered severe stretching damage to their links. The resulting slackness of the chains and ineffective positioning of the air guns has been further aggravated by loss of link material due to erosion.
Still further, it will be recognized that, inasmuch as the critical distance between the air guns is determined by the radius of the discharged air bubble, and as the different sizes of air guns used in marine seismic exploration produce air bubbles having different radii, the spacing between the air guns must vary according to the size of the guns. The positioning structure described above cannot readily be adjusted to provide different spacing for different guns--a separate such structure must be fabricated and kept on hand for each size gun which may be used in the course of the seismic exploration.
Accordingly, the present invention is aimed at providing an apparatus for effectively positioning an interdependent array of air guns which is durable and has a long service life, and which reduces shock damage to the guns in the array. Furthermore, the present invention is aimed at providing a positioning apparatus which is relatively easy to assemble and in which individual air guns may be more readily removed and reinstalled. Still further, the present invention is aimed at providing such an apparatus in which the distance between the air guns can more easily be adjusted to accommodate a range of gun sizes. The present invention is also aimed at providing a positioning apparatus which is suitable for handling on and deployment and towing from a marine seismic vessel.